To study the involvement of biological rhythm in larval ecdysis in the 3rd instar of the silkworm, the effect of temperature on the process occurring between light-off stimuli and the next ecdysis was examined. Temperature after a light pulse affected the sequential endocrine processes of ecdysis in 3rd instar silkworm larvae. There was no difference in the timing of the “head critical period” after the light pulse between larvae reared at 30°C and 26°C, but those in the 22°C group reached this stage much later. in spite of this lack of difference in the time of the “head critical period” between the 26°C and 30°C groups, the patterns of increase in haemolymph ecdysteroids and the time of the “thorax critical period” showed marked difference. © 1991, JAPANESE SOCIETY OF APPLIED ENTOMOLOGY AND ZOOLOGY. All rights reserved.